Fluid and electrolyte transport is the central function of salivary glands (SG) that is critical for oral health. A critical aspect of SG secretion is the secretion of nearly 140 mM HCO3- by the duct in which CFTR plays a central role. In addition to functioning as a Cl channel CFTR regulates the activity of several Cl and HCO3- transporters at the luminal membrane, including the newly discovered CI/HCO3 and I-SLC26 transporters (SLCTs). Another physiologically and clinically important function of the duct that has never been studied is I-transport and homeostasis. SGs are destroyed during 1311 therapy of thyroid cancers. Our contributions to this topic are a) the electrogenicity of the SLC26Ts, b) the role of the SLC26Ts in ductal HCO3- secretion and their regulation by CFTR, c) the regulation of CFTR by the SLC26Ts, d) expression of the I-transporter slc26a4 in SG cells. Recently we also found a novel ADP-activated H?7HCO3- permeable pathway in parotid acinar and duct cells. These findings led to a new hypothesis of ductal fluid and HCO3- secretion in which the bulk of HCO3- secretion is mediated by different SLC26Ts along the duct that are regulated by CFTR. In turn, the SLC26Ts suppress CFTR activity in the resting state and enhance CFTR activity at the stimulated state, which determines the final HCO3- content in the saliva. We also develop a new hypothesis of I-transport by SG cells. We will test this hypothesis in model systems AND NATIVE SG CELLS of WT, slc26a4-/- and slc26a6-/- mice in three Specific Aims: 1. Determine the transport mechanisms of the ADP- activated H?/HCO3- pathway and its role in ductal HCO3 secretion. 2. Study the role of slc26a4 and slc26a6 in SG fluid and HCO3- secretion using the KO mice. We also developed sealed ducts in primary culture in which transporters can be knocked down by dicer siRNA and in which we can measure directly fluid and HCO3- secretion. Systematic KD of SG SLC26Ts, pNBC and other HCO3- transporters will be used to study their role in ductal secretion. 3. Characterize the function of slc26a4 as an I-and HCO3- transporter and examine the role of the basolateral 2Na/l transporter NIS and the luminal slc26a4 in SG I-homeostasis and accumulation and their role in SG I-toxicity. The proposed work should advance our understanding of SG fluid and electrolyte secretion in health and disease and reveal for the first time the molecular bases for I-toxicity of SG during radiation therapy of thyroid cancers.